FIG. 1 is a drawing of the cross section of a typical KrF excimer laser chamber. The gain region of the laser is a discharge region with a cross section of about 20 mmxc3x974 mm shown as 34 in FIG. 1 with a length between elongated electrodes 36A and 36B of about 70 cm. In the chamber laser gas is circulated by fan 38 and cooled by heat exchanger 40. Also shown in FIG. 1 are main insulator 42, anode support bar 44 and preionizer rod 46.
An important use of electric discharge lasers such as KrF excimer lasers is as light sources for integrated circuit lithography. In these applications, the lasers are line narrowed to about 0.5 pm about a desired xe2x80x9ccenter-linexe2x80x9d wavelength. The laser beam is focused by a stepper or scanner machine onto the surface of a silicon wafer on which the integrated circuits are being created. The surface is illuminated with short bursts of laser pulses at pulse rates of about 1000 Hz or greater. Very precise control of wavelength and bandwidth are required to permit the production of extremely fine integrated circuit features. The operators of most stepper and scanner machines in use today operate the laser light source at about 1000 Hz, but 2000 Hz sources are being shipped and lasers with even higher repetition rates are being developed. The typical laser gas for the KrF laser is about 99 percent neon at 3 atmospheres and at a temperature of about 45xc2x0 C. At this temperature a sound wave travels about 47 cm between pulses at 1000 Hz, about 23.5 cm between pules at 2000 Hz and about 11.7 cm at 4000 Hz. Integrated circuit manufacturers desire to be able to operate their laser at any pulse rate within the operating range of the laser while maintaining beam parameters including target wavelength and bandwidth within desired specifications.
Distances between the discharge region of a typical lithography excimer laser and major reflecting surfaces within the laser chamber range from about 5 to 20 cm. Distances between reflecting surfaces in planes perpendicular to the length of the discharge region are mostly between about 5 cm to about 10 cm. Therefore, as demonstrated by a comparison of FIG. 2A showing distances traveled by sound with FIG. 1, a typical discharge created pressure wave traveling at the speed of sound in the FIG. 1 laser operating at 1000 Hz would have to make several reflections in order to arrive back at the discharge region coincident with the next discharge. At pulse rates in the range of 2000 Hz and higher, the pressure wave traveling at the speed of sound may return to the discharge region coincident with the next pulse after only one reflection.
KrF excimer lasers currently in use for integrated circuit lithography are designed for precise control of wavelength and bandwidth. Current specifications from integrated circuit makers call for control of the center line wavelength to a target wavelength, such as 248,321.3 pm within a stability range of xc2x10.1 pm. A typical bandwidth specification may be 0.6 pm, full width half maximum and 3 pm, 95% integral.
The makers of stepper and scanner machines want to tighten these specifications and also to increase pulse repetition rate to 2000 Hz and above.
A typical method of line narrowing a lithography laser is shown in FIG. 3. In this drawing the line narrowing module (called a xe2x80x9cline narrowing packagexe2x80x9d or xe2x80x9cLNPxe2x80x9d) 7 is greatly enlarged with respect to the rest of laser system 2. The laser beam exiting back end of laser chamber 3 is expanded with a three prism beam expander 18, and reflected by a tuning mirror 14 on to a grating 16 disposed in the Litrow configuration. The angle at which the light illuminates and is reflected from the surface of the grating determines the selected wavelength. For example, in this prior art laser a pivot of 40 micro radians produced by stepper motor 15 will change the wavelength of the selected light by 1 pm. The three prism beam expander shown in FIG. 3 increases the selectivity of the grating by its magnification factor which is typically about 25. A change in direction of the beam exiting the laser in the direction of the LNP can also cause a change in the wavelength selected by the grating; however, the direction change would need to be about 1 milliradian to cause a 1 pm change in the selected wavelength.
The wavelength of prior art lithography lasers is normally controlled with a feedback arrangement in which the wavelength of the output beam is sampled by an instrument called a wavemeter to measure the wavelength and the measured values are compared to a desired or target wavelength to compute a wavelength error value which is used to adjust the position of mirror 14. Typical prior art wavemeters for lithography lasers require about 3 milliseconds to measure the wavelength and to calculate the wavelength error. Another approximately 4 milliseconds are required by the stepper motor 15 to adjust the position of mirror 14. These prior art wavelength control techniques work well to correct wavelength drifts over periods longer than about 10-15 milliseconds.
Prior art KrF excimer lasers can be operated within very tight specifications even at very high repetition rates when operating at steady state, for example, continuously at 2000 Hz. However, typical operating modes for a lithography laser light source is far from steady state continuous. In a typical mode, 170 dies on a wafer may each be illuminated with 0.15 second bursts of laser pulses at a pulse repetition rate of 2000 Hz (i.e., 300 10-mJ pulses) with a 0.15 second down time between bursts and then a 9 second down time while a new wafer is loaded onto the machine. This complete cycle would take about 1 minute and would represent a duty cycle of about 42.5 percent.
Lasers operating in burst modes at pulse repetition rates in the range of 1000 Hz or greater have displayed patterns of wavelength variation over time periods of about 3 to 10 milliseconds with wavelength variations of about xc2x10.1 pm. These patterns (for the most part) have been very difficult, if not impossible to accurately predict and to date their cause has not been known. These variations are referred to as wavelength xe2x80x9cchirpxe2x80x9d. The chirp tends to increase with increasing repetition rate. Since the time required to measure the wavelength and change the wavelength with laser controls using tuning mirror 14 driven by stepper motor 15 is about 7 milliseconds, the typical chirp is history before prior art wavelength controls take effect. Because of this latency and the inability to accurately predict the chirp patterns, it has not been feasible in the past to provide active correction of wavelength chirp with the prior art wavelength control equipment.
What is needed is an electric discharge laser having provisions for active correction of wavelength chirp.
The present invention provides equipment and methods for correcting wavelength chirp in high pulse rate gas discharge lasers. Applicants have identified the major cause of prior art wavelength chirp as pressure waves from a discharge reflecting back to the discharge region coincident with a subsequent discharge. The timing of the arrival of the pressure wave is determined by the temperature of the laser gas through which the wave is traveling. During burst mode operation, the laser gas temperature in prior art lasers changes by several degrees over periods of a few milliseconds. These changing temperatures change the location of the coincident pressure waves from pulse to pulse within the discharge region causing a variation in the pressure of the laser gas which in turn affects the index of refraction of the discharge region causing the laser beam exiting the rear of the laser to slightly change direction. This change in beam direction causes the grating in the LNP to reflect back to the discharge region light at a slightly different wavelength causing the wavelength chirp.
The chirp problem can be minimized by moderating or dispersing the discharge created pressure waves or by maintaining the gas temperature as close as feasible to constant values (pulse-to-pulse). This application discloses techniques for moderating and dispensing these pressure waves. In some lasers small predictable patterns remain which can be substantially corrected with active wavelength control using relatively slow wavelength control instruments of the prior art. In a preferred embodiment a simple learning algorithm is described to allow advance tuning mirror adjustment in anticipation of the learned chirp pattern. Embodiments include stepper motors having very fine adjustments so that size of tuning steps are substantially reduced for more precise tuning. However, complete elimination of wavelength chirp is normally not feasible with structural changes in the laser chamber and advance tuning; therefore, Applicants have developed equipment and techniques for very fast active chirp correction. Improved techniques include a combination of a relatively slow stepper motor and a very fast piezoelectric driver. In another preferred embodiment chirp correction is made on a pulse-to-pulse basis where the wavelength of one pulse is measured and the wavelength of the next pulse is corrected based on the measurement. This correction technique is able to function at repetition rates as rapid as 2000 Hz and greater.
Very fast tuning mirror controls are provided for adjusting the pivot position of the tuning mirror which changes the angle of illumination on the wavelength selecting grating in the line narrowing package. These controls permit mirror tuning in a small fraction of the time interval between pulses at pulse repetition rates of 2000 Hz. This application also discloses improvements to a prior art wavemeter which permits collection of wavelength data and calculation of wavelength of each pulse in time to make wavelength tuning correction for the next pulse at pulse repetition rates of 2000 Hz or greater.
Thus, this specification discloses techniques and structured improvements to minimize chirp and also active wavelength correction techniques to correct for chirp which remains.